1. Field of the Invention
The present invention relates to vacuum switching devices, and more particularly to an improved contact structure for such devices.
2. Description of the Prior Art
When the contacts of a vacuum interrupter are opened an arc arises between the two opposed contact electrodes, which is sustained by plasma emitted from the cathode of the two electrodes due to local overheating of areas of the electrodes. The emission of plasma can be increased by so much that the resulting deterioration of the vacuum inside the interrupter causes the interrupting capability of the contactor to be substantially impaired.
It is known that the localized overheating of areas of the contact electrodes can be reduced or eliminated by the targeted influence of magnetic fields. Radially directed magnetic fields effect a spiral or approximately circular form for the base of a contracted arc, while axially directed magnetic fields produce a diffused arc. Further, it is known to combine the effects of axial and radially directed magnetic fields, where the apportioning of the respective magnetic field components is carried out in such a way that the arc is diffuse up to relatively high currents and that upon the subsequent transformation of the diffuse arc into a contracted arc, the latter is driven by the radial magnetic field component along the surface of the contact electrode.
Apart from the use of external coils, i.e, of coils disposed outside of the vacuum interrupter chamber, and the use of the contact rod to generate a magnetic field which extends into the area of the contact electrodes, it is already known to design the contact making elements associated with the contact rods in such a fashion that a coil configuration is effected and a specially-configured magnetic field is set up. A number of different configurations of contact elements are known which produce magnetic fields of different directions and intensities. Since currents constantly flow through these coil configurations, the I.sup.2 R heating produced in them contributes substantially to the total heat produced within the vacuum contactor. This added contribution can be responsible for more than half of the total heat generated in the vaccum contactor under continuous operating conditions. It stands to reason that an enlargement of the coil configuration, which determines the size of the contacts, should be made. However, this results in an increase in the volume and mass of the contact, leading to the enlargement of the drive mechanism for the contactor. It is furthermore known that, in order to keep the heat loss low, it is useful to divide the current flowing in the contact into several paths. However, the generated magnetic field is reduced in proportion to the division of the current into several paths. German Pat. No. 31 33 799 discloses a contact structure which is constructed in such a way that the continuous current of the contactor does not flow through the coil configuration of the contact; instead the coil configuration is achieved through slots in hollow disk-like elements. An internal face area of such a disk-like element lies opposite the appropriate face of the contact rod, separated by a gap. The gap is closed and contact is made by closing the contactor, the current flowing essentially through the contact rods. Upon opening the contactor, the current path between the face area of the contact shaft and the face of the hollow disk-like body is interrupted so that the total current flow runs over the surface area of the hollow disk-like body, which, with the help of the slots, leads to the generation of a magnetic field. While such a construction has advantages, it will be appreciated that it would be desirable to provide an easily manufactured, rugged arrangement for eliminating unnecessary I.sup.2 R losses while still producing the desired field pattern.
It is therefore an object of the invention to provide a contact arrangement for disabling a coil of a vacuum interrupter at the contact when the interrupter is closed and allowing current to flow through the coil when the contactor is opened.